1. Field of the Invention
This invention relates to secondary cells. More particularly, this invention relates to a secondary cell wherein the active material in the cathode is an orthorhombic alkali metal/manganese oxide material.
2. Description of the Related Art
In the formation of secondary (rechargeable) cells, it is important that the cell have a high rate of discharge, good cycling capabilities, good stability of the cathode material, high specific energy (energy per unit of weight) and high energy density (energy per unit volume). Cost of the materials used in the cell is also an important consideration. Manganese oxide is widely used in commercial alkaline batteries and primary lithium cells, in part due to its low cost, and widespread availability and has, therefore, been considered for use as active material in secondary cells. Macklin et al., in an article entitled "Performance of Lithium-Manganese Oxide spinel Electrodes in a Lithium Polymer Electrolyte Cell", published in the Journal of Power Sources 34 (1991) at pp 39-49, suggested the use of lithiated manganese oxides in spinel form, LiMn.sub.2 O.sub.4 for use in secondary lithium or lithium ion cells with solid polymer electrolytes with varying degrees of success.
Thackeray et al. in "Spinel Electrodes from the Li-Mn-O System for Rechargeable Lithium Battery Applications", published in the Journal of the Electrochemical Society, Vol. 139, No. 2 (1992) at pp 363-366, discloses the use of lithium manganese oxide spinels and defect spinels in lithium cells with liquid electrolytes. Tamscon et al., in "Li Metal-Free Rechargeable Batteries Based on Li.sub.1+x Mn.sub.2 O.sub.4 cathodes (0.ltoreq.x.ltoreq.1) and Carbon Anodes", published in the Journal of the Electrochemical Society, Vol. 138, No. 10 (1991) at pp 2864-2868, discloses a lithium manganese oxide spinel in a lithium ion cell, i.e. a cell with a carbon anode.
Ma et al., in an article entitled "Rechargeable Na/Na.sub.x CoO.sub.2 and Na.sub.15 Pb.sub.4 /Na.sub.x CoO.sub.2 Polymer Electrolyte Cells", which was published in the Journal of the Electrochemical Society 140 (October 1993), at pp 2726-2733, showed a Na/PEO/Na.sub.x CoO.sub.2 cell with performance comparable to many lithium polymer systems in terms of energy density, power density, rate capability, and cyclability.
While the use of manganese oxide-based active cathodic material is of interest, and has been explored by others, manganese oxides have several unusual (and undesirable) characteristics relevant to their use as active material in cathodes of secondary cells. Among these characteristics is the existence of a large number of phases, and a strong tendency for reduction either to manganese (II), or to Mn.sub.2 O.sub.3 with concomitant formation of sodium oxide (in sodium cells) or lithium oxide (in lithium cells) occurring. Such reactions are not completely reversible, and therefore are undesirable for rechargeable cells. Thus, for example, .gamma.-MnO.sub.2 tends to undergo irreversible reduction and is useful only in primary cells.
Reversible reductive intercalation of alkali metal ions and concurrent reduction of Mn (IV) centers to Mn (III) occurs more readily in those structures with large vacant sites that can accommodate ions. Thus, spinel-type LiMn.sub.2 O.sub.4 is thought to be more reversible in lithium cells because of its structure. However, reduction to Mn(III) is associated with a Jahn-Teller distortion of the MnO.sub.6 octahedra that forms most manganese (IV) structures; this often leads to phase changes at ambient temperatures, and causes the electronic localization that is responsible for the rather poor electronic and ionic conductivity of these materials; the flat discharge profile typical of Li/spinel LiMn.sub.2 O.sub.4 cells is evidence of such a phase change. The tendency for facile structural changes also decreases the reversibility of the manganese (IV) oxide cathode.
The synthesis and characterization of a number of different Na.sub.x MnO.sub.2 compounds or phases (Na.sub.0.40 MnO.sub.2, Na.sub.0.44 MnO.sub.2, Na.sub.0.70 MnO.sub.2, and NaMnO.sub.2) is described by Parant et at. in an article entitled "Sur Quelques Nouvelles Phases de Formule Na.sub.x MnO.sub.2 (x.ltoreq.1)", published in the Journal of Solid State Chemistry 3 (1971) at pages 1-11.
Mendibourne et at., in an article entitled "Electrochemical Intercalation and Deintercalation of Na.sub.x MnO.sub.2 Bronzes", published in the Journal of Solid State Chemistry 57, (1985) at pages 323-331, describe intercalation and deintercalation studies on a number of Na.sub.x MnO.sub.2 compounds, although they state, with respect to Na.sub.0.44 MnO.sub.2, that the occurrence of two types of manganese sites (octahedral and rectangular pyramidal) could make a hopping mechanism difficult and thereby decrease the electronic conductivity, and as a consequence, intercalation was not tested in this material.
Despite the discouraging attributes of many of the alkali metal manganese oxide compounds, it would be desirable to form a secondary alkali metal cell, using manganese oxide, in view of the low cost and availability of manganese oxide.